Media certification testing is performed for all disk drive media and is used to screen the media for defects in the magnetic layers. These defects include scratches, voids from missing media material and other media defects. Testing is generally done on special testers that include a spindle for holding and spinning the disks, head positioners (or actuators) for precisely locating the test head on the disk surface, and computers, controllers and software controlling the tester and interpreting the test results.
Generally, media certification testing is done by writing a track of bit signals with a write head or element and then reading back signal with a read head or element. If there are any defects on the disk the read back signal (output) will be compromised. In industry practice, the testing is done with one of two prior art methods.
The first prior art test procedure is referred to as Spiral Testing. It is performed with two separate or discrete heads as shown in FIG. 1. Specifically, this form of Spiral Testing uses a separate write and a separate read head that are attached to a head gimbal assembly (HGA). The HGA is then connected to separate head positioners (or actuators). These head positioners are generally very accurate positioning devices with servo and/or encoded positioning feedback. However since there is a build up of tolerances associated with using two heads and positioners there is difficulty in maintaining write track to read track alignment. Misregistration of write to read is generally considered being off-track. The alignment or centering of the disk to the spindle also contributes to this off-track problem since if not perfectly centered, the track will be written in an elliptical pattern. The industry practice to eliminate the off-track alignment problems is for the write head to be very wide, for example, 25 microns to 75 microns or more.
Spiral Testing has three primary limitations. First, the wide write test track is not feasible with perpendicular recording technology. The wide write width will not enable proper magnetization of the perpendicular media due to write saturation effects. There are also limitations with the wide write heads for longitudinal recording. Generally these limitations are due to the ability of longitudinal write head technology to properly achieve write saturation on the high coercivity media. If proper write saturation is not achieved the read back signal will be week and the ability to detect defects will be compromised. There are also limitations with the supply and availability of this older write head technology which makes them more scarce and costly. Secondly, Spiral Testing also has difficulties with testers maintaining write to read on track accuracies. If the read head is not properly aligned with the prewritten write track, the readback signal (output) will be compromised and the tester may consider this inaccuracy as a disk defect. Commonly, wide write tracks are used to overcome this difficulty. Thirdly, the tester requires two separate heads and head positioners for performing this test. The two heads are both expensive and lead to misregistration due to the requirement of aligning two separate heads.
The second prior art test procedure is referred to as Step and Repeat Testing as shown in FIG. 2. Step and Repeat Testing employs a single head with integrated write and read elements. In Step and Repeat Testing each track 201 is tested by being written and read before moving the head to another track to test. The Step and Repeat Testing method has the advantage of eliminating the tolerance problem with two separate heads and head positioners and elliptical write track patterns. Furthermore, this testing method allows the write element structure to have a narrower write track to better enable proper write saturation. Lastly, there does not need to be two separate heads and scanners. This is because the same head writes and reads the test data and includes both a read and a write element.
However, the Step and Repeat Testing method has a disadvantage versus Spiral Testing in testing time and throughput. Step and Repeat Testing requires time to write one track and then read that same track during the next revolution. After reading, the head positioner needs to move the head to another track, which requires additional time to move and settle. Then this write-read-move cycle is repeated. Thus, Step and Repeat Testing is between two to three times longer than Spiral Testing. This additional time has a big impact to media manufactures due to the additional testers and space required to overcome the slowness of the method.
What is needed is an effective testing procedure that is both fast and efficient.